1. Field of the Invention
The present invention relates to composite grinding plates which have a contact surface intended to withstand abrasion. Such plates may be used for grinding ceramic powders or for the defibration and refining of paper pulp.
2. Description of the Prior Art
In grinding machines for ceramic powders and defibration or refining machines for paper pulp, such as those described in patents U.S. Application No. 3 117 603 or U.S. Application No. 4 423,845 for example, two plates rotating in relation to each other each bear on an assembly of sectorial plates laid out in a circular ring, the working faces of the plates being equipped with generally radial grooves and being made of an abrasive material, for example alumina, zirconium oxide or silicon carbide.
The working face of the grinding or defibration plate is generally flat. The grooves are the only irregularity on the plate surface, and their shape is carefully chosen to obtain the paper pulp defibration or ceramic powder grinding effect. The material inserted between the plates must be able to flow along the radial grooves, and the shape, spacing and direction of the grooves determine the size of the paper pulp fibres or the ceramic powder grains to be obtained. The plate must thus have very high dimensional and positional accuracy.
Research has shown that grinding machines equipped with plates such as those disclosed in document U.S. Application No. 3 117 603 experience operating disturbances when the plates are made of hard abrasion-proof materials. Disturbances tend to increase with time during machine operation. In particular, a decrease in the grinding efficiency is observed.
Document U.S. Application No. 4 423 845 states that surface irregularities should be deliberately provided, consisting of non-contiguous studs of hard material separated by less hard areas. Uneven wear takes place during plate rotation making the hard studs project, momentarily increasing the grinding efficiency. However, wear continues rapidly, reducing the grinding efficiency.
Other grinding machines are known, for example such as described in documents EP-A-0 227 879, AU-A54 636 or GB-A-2 168 988. The grinding wheels are made of grains of hard material bonded in a polymerized resin. In a grinding machine, the grinding surface is plane or conical with no radial grooves, and consists of small asperities designed to tear out small parts of the material to be ground. These asperities, consisting of grains of hard material, must project above the general surface of the grinding wheel, and the grinding surface is generally regenerated by the gradual tearing out of the asperities and the creation of other asperities due to the fact that the surface has a discontinuous hardness creating preferential resin wear zones, allowing the harder zones forming the asperities to project progressively. The grinding surface thus has an abrasive power.
The invention is a result of the observation according to which abrasive linings used in known grinding or defibration machines have uneven surfaces and are subject to unacceptable wear which is probably due to erosion by cavitation. Consequently, despite the use of hard materials, operating disturbances and reduced grinding efficiency very significantly reduce the life and advantages of such grinding plates.
The present applicant has made very detailed observations of abrasion-proof surfaces made from melted tungsten carbide. Such observations have shown the appearance, in some cases, of relatively fast wear of such surfaces despite the use of particularly hard hardfacing materials apparently capable of withstanding abrasion.
The applicant has observed that abrasion-proof surfaces based on melted tungsten carbide made using known techniques, for example by melting a weld bead, have uniformity defects in the distribution of hard components in the abrasion-proof layer. Such uniformity defects may exist when the abrasion-proof layer is manufactured or may appear during the subsequent use of the surface.